In a V-belt CVT which is mounted in a vehicle, a V-belt is held between a primary pulley and a secondary pulley, and motive power is transmitted by a contact frictional force between the V-belt and the pulleys. The groove widths of the primary pulley and the secondary pulley are controlled by means of oil pressure.
The control device for a continuously variable transmission which is disclosed in Japanese Patent Application Laid-Open No. 2000-18347, published in Japan in 2000, sets a pressure supplied to the primary pulley and a pressure supplied to the secondary pulley when the pulley ratio (i.e. speed reduction ratio) of the CVT is changed. The control device calculates a target secondary pulley pressure in accordance with a target pulley ratio and an input torque, and obtains a target primary pulley pressure in accordance with the calculated target secondary pulley pressure and a target shift speed. When the target primary pulley pressure is lower than or equal to a critical pressure of the V-belt slippage, the control device re-calculates the target secondary pulley pressure in accordance with the critical pressure of the V-belt slippage on the primary pulley. The target pulley ratio is achieved by this control, without generating slippage of the V-belt.
However, this conventional control device for a continuously variable transmission provides a predetermined margin when setting the target pulley pressure. For this reason, the target pulley pressure is needlessly set higher when the CVT is at high gear in which the target pulley pressure may be low. Here, high gear of the CVT means that a pulley ratio is less than 1, and low gear of the CVT means that the pulley ratio is higher than 1.
Proceedings No. 04 CVT-39 of CVT congress 2004 which is held in 2004 suggests changing the margin of the target pulley pressure in accordance with the pulley ratio. This eliminates setting the target pulley pressure high needlessly.